1. Field of the Invention
This invention relates to a developing toner for use in developing an electric or magnetic latent image in the electrophotographic method, electrostatic printing method, or magnetic recording method.
2. Description of Prior Arts
For the toner to develop an electric or magnetic latent image, there have generally been used so far pulverized material having a particle size of from 5 to 20 microns or so prepared by mixing and kneading under a molten condition a binding resin such as polystyrene, a coloring matter such as carbon black, and depending on necessity, an additive, followed by pulverizing the mixture. Such dry-type toner is generally manufactured by a dry method which uses on solvent. On the other hand, a wet method, which utilizes a solvent as in a micro-capsule toner which has been actively studied in recent years, is seldom used for the reasons that manufacturing steps such as dissolution into the solvent, recovery of the solvent, etc. become complicated, that the toner having a particularly superior capability to that obtained by the dry-type is difficult to be manufactured, and others, hence this method is scarcely used at present.
In recent years, an image fixing system using a pressure-applying roller has been developed strenuously from the energy-saving standpoint, in accompaniment of which development of the toner having a pressure-fixing property is becoming an important subject. However, the toner, at the present stage, requires a linear pressure as high as 30 kg/cm or so between pressure rollers in order to provide a satisfactory image, and application of such high pressure brings about various troubles such as curling, lustering, wrinkling, etc. in an image bearing member, and also necessitates rigidness in the fixing device, which inevitably leads to increase in size of the apparatus and its manufacturing cost. On account of this, development of a toner has been eagerly desired which is capable of being fixed at as low a pressure as possible. That is, when a soft material such as, for example, polyethylene wax, etc. of a low molecular weight is used, a low pressure fixable toner can be obtained. However, various problems still remain with such toner. For example, in case the copying operation is continuously done on paper having different sizes, i.e., when the copying is to be done on paper having a narrow width, after which it is done on paper having a broad width, the toner on a portion which did not participate in copying on the paper having narrow width deteriorates to bring about lowered image density, and further causes agglomeration and caking of the toner particles during the development, and insufficient cleaning.
Heretofore, there has been proposed a method, in which emulsified small particles are together formed into size-enlarged particles to provide a void structure among the small particles so that the pressure fixing property of the toner is improved due to fragility of the void structure (vide: laid-open Japanese patent application No. 53-48740). This proposed method is disadvantageous in that the toner having stable particle size, strength, and porosity is difficult to obtain. Further, developing property of the toner is apt to be subjected to influence of humidity due to residual surfactant.
British Patent Specification No. 1192920 also discloses a method for producing granular ink having voids therein, which method is to remove a soluble solid substance from a spherical ink product obtained by spray-dry method with use of a solvent. However, with the spray-dry method, kind of binding resins applicable for the purpose is limited. Moreover, the spray-dry method is not suitable, in particular, for a pressure fixing substance such as ethylene type polymers, etc. Furthermore, with the spray-dry method, the produce assumes a true spherical shape, on account of which its pressure-fixing property is very poor, even if the soluble solid substance is removed from the product. In addition, with the spray-dry method, when a component insoluble in the solvent (e.g. elimination compound as explained later is insoluble in the solvent for the spray-dry method is present in a large quantity, various problems would inevitably arise such that adequate size distribution, shape, and composition of the toner particles are difficult to be obtained, and the void structure and porosity of the porous toner are difficult to be controlled, because difficult to be obtained the void structure tends to be readily formed by evaporation of the solvent at the time of the spray-drying.